1. Field of the Invention
The present invention relates to a base station, a mobile station, a radio communications system, and a radio transmission method that use spread spectrum and chip repetition.
2. Description of the Related Art
Development of the fourth generation mobile communications method that is a mobile communications method of the next generation of IMT-2000 (International Mobile Telecommunication 2000) is progressing. The fourth generation mobile communications method is desired to flexibly support versatile cell environments, i.e., from multi-cell environments including cellular systems to isolated cell environments, such as hot spot areas and indoors, while enhancing frequency use efficiency in both types of cell environments.
As a candidate for the radio access method to be applied to a link from a mobile station to a base station (an uplink) in the fourth generation mobile communications method, direct spread spectrum code division multiple access (DS-CDMA: Direct Sequence-Code Division Multiple Access) is leading from the viewpoint that it is especially suited for cellular systems. According to DS-CDMA, symbol sequences to be transmitted are multiplied by a spreading code such that a spread spectrum signal is obtained, which spread spectrum signal is transmitted through a wide frequency bandwidth (for example, Non-Patent Reference 1).
Reasons why the DS-CDMA is suitable for the multi-cell environment including cellular systems are as follows. First, as compared with radio access methods using a great number of subcarriers such as OFDM (orthogonal frequency division multiplexing) and MC-CDMA (Multi-Carrier Code Division Multiple Access), the ratio of peak power to mean power can be made small. Accordingly, the power requirement of the mobile station is small, which is an important property of the DS-CDMA.
Second, while reduction of the transmitted power required by a synchronous-detection recovery using an individual pilot channel is effective in the uplink, the pilot channel power of DS-CDMA per carrier is greater than that of OFDM, MC-CDMA, etc., for the same level of pilot channel power. Therefore, highly precise channel estimation is obtained, and the transmission power can be made small.
Third, in the multi-cell environment, DS-CDMA can use the same carrier frequency in an adjacent cell, since interference from the adjacent cell (“adjacent cell interference”) can be reduced by the spread spectrum gain obtained by spreading the spectrum of the signal. For this reason, a repetition of “1 cell frequency” is easily realized, i.e., all the available bandwidth is repeatedly assigned to each cell. Accordingly, frequency use efficiency of DS-CDMA is high as compared with TDMA (Time Division Multiple Access) wherein the “adjacent cell interference” is avoided by splitting the available frequency bandwidth, each cell being assigned only a split segment of the frequency bandwidth differentiated from cell to cell.
However, since DS-CDMA is a radio access method suitable for multi-cell environments, problems are anticipated as follows. That is, the advantage of DS-CDMA in that the influence of the “adjacent cell interference” is reduced by spread spectrum is not an advantage in isolated cell environments, such as small hot spot areas and indoors where “adjacent cell interference” is not a concern. For this reason, in order to realize the same level of efficient frequency use by DS-CDMA as TDMA, it is necessary to accommodate a great number of signals.
For example, where each mobile station transmits by multiplying a spread spectrum code of the spreading factor SF by a signal to be transmitted, information transmission speed is 1/SF. Here, in order to realize the same frequency use efficiency as TDMA, the number of signals that DS-CDMA needs to accommodate is equal to SF. However, in the radio propagation environment of an actual uplink, multiple-access interference (MAI: Multiple Access Interference) becomes dominant. MAI is interference to a signal of a mobile station by another mobile station, MAI being originated by a difference of propagation conditions among mobile stations to a base station, for example, change in a propagation delay time and a propagation path. Consequently, the frequency use efficiency normalized by the spreading factor is reduced to about 20% to 30%.
In order to reduce the MAI, a radio access method called IFDMA (Interleaved Frequency Division Multiple Access) is being studied (for example, Non-patent Reference 2). According to IFDMA, information symbols are rearranged by applying symbol repetition so that a certain symbol pattern is generated, a unique phase of a mobile station is multiplied by the symbol pattern, and the multiplied signal is transmitted. In this manner, signals from mobile stations are arranged not to overlap each other in the frequency domain, and MAI is reduced.
Another approach for reducing MAI is being studied, wherein transmission timing is controlled so that the frequency use efficiency is raised (for example, Non-patent Reference 3). FIG. 31 is a timing chart showing timing of signals being received according to the conventional technology with and without transmission timing control being applied. As shown in FIG. 31, when timing control is not applied, signals transmitted by mobile stations 210 through 230 arrive at a base station 110 with different receiving timings due to different propagation delay times to the base station 110. Then, the transmission timings of the mobile stations 210 through 230 are controlled such that the signals transmitted by the mobile stations 210 through 230 arrive at the base station 110 with the same timing as shown in FIG. 31. If an orthogonal code is used as the spreading code at this time, received signals from the different mobile stations at the timing are orthogonal to each other, and the multiple access interference (MAI) is reduced. In this manner, the frequency use efficiency is raised.
Further, another approach of suppressing the multi-path interference is being studied, wherein a receiving unit carries out signal processing to a received signal influenced by multi-path interference. Typical examples using this approach are a multi-path interference canceller (for example, Non-patent Reference 4) as shown by FIG. 32, a chip equalizer (for example, Non-patent Reference 5) as shown by FIG. 33, and a frequency domain equalizer (for example, Non-patent Reference 6) as shown by FIG. 34.
According to the multi-path interference canceller shown in FIG. 32, a signal component causing the multi-path interference is estimated and generated (multi-path interference replica) by a multi-path interference signal estimating unit 351, and the multi-path interference replica is subtracted from a received signal by a multi-path interference signal removing unit 352. Thereby, the influence of multi-path interference of the received signal is reduced.
According to the chip equalizer shown in FIG. 33, a channel matrix that expresses the amount of change that the signal receives through the propagation path is generated by a channel matrix generating unit 361, a weighting factor that reduces the multi-path interference is deducted from the matrix by a weighting factor estimating unit 362, and the weighting factor and the received signal are multiplied by a chip equalizing unit 363 (this operation is called chip equalization). Thereby, the influence of the multi-path interference is reduced.
According to the frequency domain equalizer shown in FIG. 34, the received signal is converted to a signal in the frequency domain by a time-frequency conversion unit 371, a weighting factor that reduces the multi-path interference is generated by a weighting-factor estimating unit 372, the weighting factor and the received signal in the frequency domain are multiplied by a frequency domain equalizing unit 373, and a frequency-time conversion unit 374 converts the signal to a signal in the time domain. In this manner, the influence of the multi-path interference is reduced.
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